Engine systems may be configured with a split exhaust manifold wherein exhaust from some cylinders is directed to the exhaust passage via a first exhaust manifold and cylinder exhaust from different cylinders are directed to a common tailpipe via distinct exhaust passages.
One example of a split exhaust engine system is shown by Olofsson in EP 1127218 B1. Therein a multi-cylinder system with an exhaust driven turbine is used to power a compressor. The split exhaust system connects a first exhaust valve from each cylinder through a first exhaust branch to the turbine, while a second exhaust valve from each cylinder bypasses the turbine and directly connects to a second exhaust branch leading to an exhaust catalyst, located downstream from the turbine. Adjustments in valve timing can be used to control flow of air into the engine via intake valves and control flow of exhaust energy via the first exhaust valve to the turbine and via the second exhaust valve to the exhaust catalyst in response to changes in engine speed.
In still other engine systems with a split exhaust manifold, a first exhaust valve of a cylinder may be opened earlier in an engine cycle to deliver exhaust mass flow from an initial portion of an exhaust phase to a turbine, while a second exhaust valve may be opened later in the engine cycle to deliver exhaust mass flow from a latter portion of the exhaust phase directly to an exhaust catalyst, bypassing the turbine. In this way, by directing exhaust gases away from the turbine during the latter portion of the exhaust phase, the pumping penalty associated with high turbine backpressure may be reduced.
However, the inventors herein have recognized potential issues with such split exhaust systems. One shortcoming may be reduced engine efficiency due to significant energy losses in engine exhaust manifolds. As an example, a significant fraction of the exhaust energy delivered to the turbine may be lost due to an extensive network of exhaust manifolds between the exhaust valves and the turbine, and may lead to reduced engine efficiency.
Another shortcoming recognized in split exhaust systems may be reduced capability to control turbine speed, generator noise, vibration and harshness (NVH), or over-temperature of components such as cylinder head, exhaust valves, exhaust manifold, turbine, catalyst, etc. As a result, during engine operation turbine speed may increase above threshold levels which when left unchecked may lead to sub-optimal performance of the system.
Thus in one example, some of these issues may be at least partly addressed by a method for an engine, comprising: delivering exhaust from a first exhaust valve of all cylinders of a first cylinder group to a first scroll of an exhaust turbine while delivering exhaust from a first exhaust valve of all cylinders of a second cylinder group to a second scroll of the exhaust turbine; and delivering exhaust from a second exhaust valve of all cylinders of the first and second cylinder group to an exhaust catalyst, while bypassing the turbine. The method may further comprise, in response to turbine speed greater than a threshold speed, selectively deactivating the first exhaust valve of one or more cylinders of the first and second cylinder group. In this way, by grouping cylinders, the exhaust manifold volume between an individual cylinder and the turbine can be reduced, allowing for efficient delivery of exhaust energy to the turbine and minimizing energy losses. By selectively deactivating the first exhaust valve of one or more cylinders of the first and second cylinder group, an amount of exhaust mass flow to the turbine may be reduced and turbine speed may be controlled to modulate generator output and/or to avoid turbine or generator failure and/or to reduce generator NVH and/or to improve engine efficiency.
As one example, an engine may include a plurality of cylinders organized into a first and a second cylinder group. Responsive to a turbine speed greater than a threshold turbine speed, one or more cylinders may be selectively deactivated. For example, the first exhaust valve of one cylinder of the first cylinder group and one cylinder of the second cylinder group may be selectively deactivated for a specified duration. The selective deactivation of the first exhaust valve limits exhaust flow to the turbine, reducing turbine speed and reducing generator output and reducing generator NVH to threshold levels. The one cylinder of the first and the second cylinder group may be selected based on their position on the engine block and/or their firing order so that while reducing the turbine speed, NVH issues are reduced. In other examples, as the turbine speed exceeds the threshold turbine speed, a larger number of cylinders of the first and/or second cylinder group may be selected and their corresponding first exhaust valves may be selectively deactivated until the turbine speed is within a desired range.
The approach described here may confer several advantages. For example, the method provides improved capability for delivering exhaust energy from engine cylinders to the turbine. Consequently, the approach reduces energy losses in the engine exhaust manifold. Furthermore, the approach allows control of turbine speed over a wide range of engine operating conditions. By controlling and limiting turbine speed (and therefore generator speed) to a desired speed (or speed range), elevation in generator output and generator NVH and rise in component temperature may be limited to desired threshold levels, improving engine efficiency. In addition, adapting an exhaust system with a smaller network of exhaust manifolds between the cylinder and the turbine limits energy losses in the exhaust system and ensures improved generator output and therefore improved overall engine/generator efficiency and performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.